Tetrafluoroethylene polymer bonded heat-resistant fabric



Jan. 17, 1956 K. F. RICHARDS 2,731,063 TETR UOROETHYLENE POLYMER BONDED EAT-RESISTANT FABRIC Flled Sept 25 1950 ATTORNEY United States Patent` 1,.

2,731,068 s .TETRAFLUOROETHYLENE iPOEYMERi-BONDED 4HEAT'RESISTALNYI :FABRIC Kurt Fred Richards, Park Forest, `Ill., assigner toEJ-"L du .Pont Vde INemours an"`d Company, Wilmingtom Del., fa

:corporationfofxDelaware .ApplicationiSepteniberS23,s1950,- Serial No.-1.-86,`423 14 Claims. f(Cl. 1154-25) This inventionl relates Atosnew polymeric rmaterialastructures and, more particularly, .to laminated :substrates impregnated with polymeric-@materials :comprising tetrailuoroethylene'unitsrintpreponderanttamountxandza proc- A-ess of ,preparinggsame Polymers :of tetrafluoroethylene iandfcopolymers ethere- :of have gfound a .uniquegplace :in the :field Zioffelectrica'l insulation where;high "temperatures which'nvillrchar most known organic Yinsulating-zmaterials Aare encountered. In Athe construction of electrical motors, unsupportedjrfllms of polytetrauoroethylene vhavezbeenused as .slot lliners ,to provide .insulation `between :the lconductors and :the fground. The slo.tlinersfmnst:beresistantztofthigh temperatures' and must `have va :certain degree-.Lof'rri'gidityfas lwell as tcld-Iilow :resistance and zicut=through `:resistance when aconductorfispressed against the rinslation. ,llt has been found ,.desirable, ihowever, fin ithisrspecicield and others, to have available improved structures comprising such polymeric 4materials Aand having still greater -mechanical strength and better electricalproperties.

.Laminates of polymeric 4.materials comprising .tetratluoroethylene unitsin preponderanLamount/with :other l materials have been suggested for these purposes, .but those heretofore availablelackithe desirblepropertes set'forth above. For instance, suchjlam'inates may "be made by coating the surfaces to be joined with..a sus- '.pensoid of a polymeric l*material "comprising tetraliuoro- -ethylene units in preponderant vamount and, before the suspens'oid is dry, placing the coated 'surfaces acetoface andheatingthe 'assembly 'at"212' 'andfthen subsequently at `or above 621 F. Although'his'method produces .a .firm .bond-between. the y.treatedsurfaces,when the material to .bejoined is .a.fabric,..it'.has,notbeengpossible to forman assembly .'free V.of voids ...or .air,paces between .the threads of `the..fabric,` because .the J4.heat .ap- .plied -to :dissipate the. suspendingamedium xfor ithegpolyrneric .material ,produces lla,.,ga s which makes ...the Qbonding medium porous'. lln another method, v.apreformed -iilm of polymeric..material .comprising `..tetrafluoroethyl- -ene .units in ,.preponderant :amount fis .inserted between Atwo ,plies of nnimpregnatedglass `fabricanrLheat .and

`.pressure .areapplied A`While the...resulting, product.may

be suitable for certain .purposes numerous airspaces vwill exist between the.layers-..and..thelaminaetareteasily .separated -into 4the original plies.

Consequently, suchE prior .laminates are unsuitablesfor use wherehigh dielectricistrengthnisfrequiredasfin .slot liners for electrical motors,I or `:where .improved :abrasion resistance or improved mechanical strength is `desired.

yA Acopenrling .application'of .Philip F. Sanders, Serial No. 186,446, ledSeptembenZS,1.1;950, .disc/losesa process of producing.such-improved vv.laminates .by;.arranging in :superposed .relation .agplurality .of .dry plies -,of..fabric .impregnated withxa polymericv materialcomprising tetra iluoroethylene .units in preponderant .a1nount..ar1d lheat- `ing .the assembled plies .under-v pressure.. at the fusiomtem- `perature of .the polymeric material. This ,producesa unitary; s'tructure..in @which fthef-fabnicmlies 'fare rcornplete- 'ly encased. or een-:bedded .within Ih'e mass of polymeric material. Howevenzzthis :Sanders processii essentially ai 'batch-,procesa and rtherexas;heretoforeizbeenerlo.;Prac- :tical wayxofproducing :laminates inf Athe limaterials..h'ene 2,731,065 .Patented Jan. 17, 1956 ."2 .indescribedona continuous lbasis or' in extensive lengths` 'Itiis ythereforefaprimary object of 4this [invention to .provide continuous process ,for preparing .laminates of 'fabric `.and polymeric .material comprising .tetrauoro- .ethylene units in ,preponderant amount.

.llt ,is another object to provide a .continuous process "for producing a ,unitary .structureof extensive .length of .a plurality.of,glass asbestos, and/.onmetal fabric .plies and such `polymeric material.

It is a -further object to provide a .process ,for ,producng fa unitary structure Lin which .suchgfabric plies are embedded withinl a ...mass of `polymeric material.

it is a still qfurther object ,to provide aunitary structure of extensive length .f.a plurality of.. fabric plies .encased in a polymeric `material .comprising .tetrailuoroethylene units in .,prepon'derant amount.

It is an yadditional '..object .to provide va .laminate of extensive length of glass, asbestos, and/or metal fabric `and vsuch Apolymeric `material .in which there are substantially no voids ,between .the (threads ofthe fabric or air spaces in the'laminat'e.

lt is' another object to providealaminateof extensive length of -fabric and suchpolyme'ric materialhaving im proved 'dielectric strength, -mechanical ,.strength, and abrasion resistance.

It is another object .togprovide ya -laminate of -.extensive length offabric and-.such polymeiicmaterial which maybe used as .fslot"1iners`l`t`or .electrical motors.

it, is a specitcbjectto,provide alaminate .o'fextensive 'length vo'f glass 'fabric .and .polymeric material ycomprising tetrailuoroethylene. units in preponderant amount.

.Otherobjec'ts Willbe apparent .as vthe description `proceeds. l

These objects are accomplished by placing .togetherin superposedrelat'ion*two-.or more continuous llengths or plies of'fabric, such as g'lass, asbestos, and/onmetalfabric, impregnated with a` polymeriomaterialcomprising tetraliuoroethylene units ingpreponderant amount, .theimpregnant being thoroughlydry, subjecting .the-,plies .to -.sulicientpres'sure -at'.a temperature, considerably below vthe fusion temperature o'f thecplymericmateril to cause the separtejlies 'to '.be compacted.,and `coherent, Aand fthen heating the coherent Vplies at atemperature .above .the fusion. point of thefplyme'ric.y material in t'heabsence .of any substantildpressure tojcausey the-polymeric material to'become welded together.

'It is surprising and Wholly unexpected ,theta .unitary v structure canbe produced .by rstepressing .enrolling separate p`1iestoge'ther.at a-.temperature considerably :below thefusion point of the polymeric material, and .then.sub

`sequently heating,`in the absence ofrpressure, at orabove the fusion temperature. Y The pressing or rolling apparently causesthe .polymeric material to flow so that thepliesare intimately compacted andhave sullicierit. coherence tofpermit sbsequentprocessing without becoming separated, .andthe subsequentheating causes the impregnant of adjacent plies to become welded together to form'anintegral unitary structure of the.fabric ..plies-;and thegpolymer substantially.` free .of voids. .The .fusion of :the `,polymer is :so :complete .that 1 the fabn'c isactuallyernbedded 'orfencasedfin amass of thepolymer. It is preferred that the fabric be impregnated with .a high solids colloidal dispersion of polytetrauoroethylene, such as'those'disclosedin'U. S. Patent'No. 2,478,229, issued August'9, 1949, 'to Kenneth L. Berry, or the dilute suspensoids disclosed in copending application SerialNo. z^/" .l3,385,r filed -N'ovembfei-SO, 1946, 'by Malcolm M..Ren frew, nowPatent.2,534,058, vand concentrated by the electrodecantation method describedin copending applica- -tion Serial No. 783,389; led'OctoberSl, `1947,"by..1C. K. Ikeda, nowabandoned In any "case, 'the v'dispersed par- .ticles ofnpolymeric material shouldbe .of.such..=size and quantity that they readily permeate the fabric and fili the interstices thereof.

Any method of imprcgnating the fabric before its use in the present invention issuitable, provided that it produces a fabric in which all of the interstices are completely filled. A preferred method of accomplishing this result is disclosed in copending application Serial No. 86,606, tiled April 9, 1949, by Philip F. Sanders, now Patent 2,539,329.

For a better understanding of the nature of the objects of this invention, reference may be had to the following description and accompanying drawings in which:

Figure 1 is a schematic arrangement in cross-section of a suitable apparatus for preparing a tiexible laminate to be wound in rolls after it emerges from the fusing tunnel.

Fig. 2 is a schematicarrangement in cross-section of a suitable apparatus for preparing a relatively stiff or semirigid structure which is too stiff to be wound up in roll form, in which case the laminate is cut to the desired length as it emerges from the fusing tunnel.

Fig. 3 is an enlarged cross-section of a continuous unitary structure produced by lamination of the plies as shown in Fig. 1.

As shown in Fig. 1, the ends of two rolls of previously impregnated fabric (the impregnant being thoroughly dry) are arranged in superposed relation and fed into the nip of the iirst set of unheated pressure rolls. This first set of rolls has a steel roll on the top and a paper counter roll on the bottom. After passing through the first set, the superposed plies pass through the second set of rolls in which the steel roll is on the bottom and the paper roll on the top, then through the third set which is like the first, and then through the fourth set which is like the second. As the ends of the superposed plies emerge from the last set of rolls, the continuous coherent assembly is passed through a heating tunnel maintained at a temperature high enough to heat the polymeric material impregnant to at least its fusion temperature, and, as it emerges from the heating zone, it passes over a watercooled roll, and the finished, firmly bonded product is wound up on a roll.

In the modification shown in Fig. 2, the ends of three rolls of previously impregnated fabric are arranged in superposed relation and fed into the nip of `pressure rolls and through a heating zone similar to those shown in Fig. l. As the firmly bonded. unitary structure emerges from the heating zone, it is passed over a water-cooled table. Since a plurality of plies of this type may result in a fairly rigid structure, it is not always -feasible to wind up the product on rolls. For this reason, the continuous product passing over the water-cooled table may be cut into any convenient lengths.

It will be obvious from the herein description that there is no limitation on the length of the plies which may be laminated in the practice of this invention. As a roll of single ply material comes to an end, a new roll may be joined thereto and the process kept in operation without interruption.

The following examples are illustrative of the invention. Throughout the specification the percentages are all expressed on a weight basis.

Example 1 i A continuous length of a standardsquare weaveglass fabric, identified as Owens-Corning Fiberglas Corporations ECC-127, and having the lfollowing characteristics:

Thickness (inches) .007 Ounces per square yard 6.00 Yarn:

Warp 4503/2 Filler 450% Thread count per inch:

Warp 42 Filler 32 was given three dips in an aqueous suspensoid of poly tetrafluoroethylene. The concentration of the suspensoid for the rst and second dip was 40% solids, and, for the third dip, the concentration was 60% solids. After each dip, the saturated glass fabric was passed through a heat zone having an air temperature of approximately 180 F. at the entry port and 550 F. at the exit port to evaporate the aqueous suspending medium for the suspensoid. After this heating, the impregnant was fragile and contained microscopic mud cracks, although the polytetrauoroethylene particles adhered to themselves and to the glass fabric sufficiently to permit the impregnated fabric to be wound on a 3-inch diameter roll without damaging the material.

The average thickness of the glass fabric after impregnating and drying was .014 inch to .012 inch.

The continuous length of dry, unfused, impregnated glass fabric was cut into three pieces which were then arranged in superposed relation, and the whole assembly was passed between a series of pressure rolls, as shown in Fig. 2, at room temperature. This compacted the plies and'made them adhere to each other. The assembly was then passed through a heat zone having an air ternperature of 700-800 F., where the impregnant was heated to at least its fusion temperature of 621 F.

After the laminated assembly was cooled, it was cut into suitable lengths having substantially no voids in the entire structure. There were no mud cracks in the fused polymeric material, and the individual plies had been firmly bonded into an integral unit which could not be separated into its original components.

Example 2 A continuous length of standard square weave glass fabric, identified as Owens-Corning Fiberglas Corporations ECC-112, and having the following characteristics:

Thickness (inches) .003 Ounces per square yard 2.09 Yarn:

Warp 450% l Filler 4501/2 Thread count per inch:

Warp 40 Filler 39 was given three dip coats of the aqueous polytetratluoro ethylene suspensoids employed in Example 1. The impregnating and drying operations were carried out in the same manner as described in Example l, except that the mud cracks in the impregnant, formed during the drying operation, were sealed by giving the dry, unfused polytetratluoroethylene-impregnated glass fabric four passes between a steel pressure roll heated to about 250 F. and a paper counter roll. The pressure on the axis l of the steel calender roll against the paper counter roll was 40 tons and the speed was 354 yards per hour. The rst and third time the impregnated glass fabric was passed between the pressure rolls one side was in contact with the steel roll and the second and fourth time the other side was in contact with the steel roll. The thus calendered, unfused, impregnated glass fabric was free of mud cracks and voids between the separate threads of the fabric.

The continuous length of dry, unfused, impregnated glass fabric was cut into two pieces of equal length and a third'piece of somewhat shorter length. These were then arranged in superposed relation, the shorter piece being placed between the two longer pieces with all three pieces alined at one end, and the whole assembly was passed between a series of four sets of pressure rolls, each set consisting of a smooth surface, steel roll, heated at about 325 F., pressing on a smooth paper counter roll as shown in Fig. 1. The sets of rolls were so arranged that in the rst and third the steel roll was on the top and in the second and fourth the steel roll was on the bottom. As the laminate assembly passed through the avenues series of rolls, 'the heated 'steel `rolls lwere thus alternately on one fside and fthen on 'the other 4side ofthe assembly.

The'above-described lamina-te assembly produced a three-ply structure vfor the lirst part of the -length of superposed yplies and a two-ply structure `for the lat-ter part. After rolling, the coherent laminated structure was fused by passing lit through a heat zone having an air l'temperature of approximately 750 F., vat atmospheric pressure, tand then cooled to room temperature.

Both the two-ply and the 'three-'ply constructions were formed into a Ihighly abrasion-resistant, integral unit which -could not be separated into the -original plies. The laminates were tested for dielectric strength according to AST-M Method D14944, Short Time Test vin Air, using a -60 vcycle alternating current and V4 inch ibrass electrodes with the following results:

A' Avg: Min. Max. i VTotal Volts Volts Volts f Thick- 1p? per Y per ness,

il Mil Mil Inches 2-Piy Laminate 76o 61e 850' v .joio 3-Ply Laminate '778% .654 '895 :016

x.Average often readings.

` Example 3 A continuous .length of a standard square Weave glass fabric, identified as Owens-Corning AFiberglas Corporations ECC-M58, and having the following characterwas given three dip coats of Vthe suspensoids described in Example l and in a similar manner. .The amount of dry polymer deposited corresponded to .approximately 4.0 ounces per square yard. After drying, fthe treated glass fabric was calendered on each side to close the mud cracks formed during the ,drying step in a similar manner to that described lin Example 2. Two pieces were then lcut from `this continuous, dry, unfused, impregnated fabric, were l.placed together in superposed relation, and were laminated in a continuons manner by feeding 'them into a series of four sets of pressure rolls followed 'by a fusing operation in which the laminatedmaterial wassubjected to an 'air temperature of 750 F. for a period of '3 minutes with an apparatus similar to that shown in lFig. l. Each set of pressure rolls 4consisted of a steel roll .heated to approximately 325 F. and a paper counter `ro'll of the type commonly used in calendering and embossing of coated textile materials.

The average thickness of the resulting exible, unitary structure was .006 inch, and the average dielectric strength was found to be 1,000 volts jper mil when `tested in the same manner as that described in 'Example 2.

Example '4 .Two more continuous lengths or plies 'cut from ,the dry, unfused, impregnated and calendered glass yfabric described in Example 3 were arranged in superposedrelation, and were placed in a hydraulic press where they were subjected to a pressure of approximately 10,000 lbs/sq. in. at a temperature of 200 l"for a period of 3 minutes. This operation compacted the two plies into intimate contact with cach other, and substantially no a'ir spaces or voids existed in the assembly. The separate plies adhered to each other,.'but vthey could be readily separated. The assembly 'was next 'subjected to an air temperature -of 700450 F., at'atmosrheric pressure, for

a suiiicien't length -oftimeto heat the polytetratluoroethylene 'to atleast its fusion temperature (621 F.) and cause the plies `to become welded together to form an integral unit. After cooling to room temperature, the two plies were so rmly welded together that they vcould not be separated into the lvoriginal plies. The product had an average 'thickness of 006 inch, was `flexible and highlyabrasion-resistant, and had good dielectric strength.

Example 5 A continuous length of non-woven asbestos fabric weighing 2:8 ounces .per 4square yard and having an approximate thickness of .008 inch to .(lrll) inch was dip coated by passing it through ,a 50% aqueous suspensoid of polytetrauoroethylene to 4.saturate and impregnate the fabric. The impregnated fabric was -then heated at approximately 230 vto evaporate the water. The weight of dry polymer deposited in the `fabric was 4.5 ounces per square yard. The continuous length Vof the dry saturated or impregnated -asbestos fabric was passed between pressure rolls heated at approximately 270 to smooth yand compact the material after which it had a thickness of approximately 008 inch.

The continuous length of dry, unfused, impregnated asbestos fabric was cut into three pieces which were then arranged in superposed relation, and ythe Whole assembly was passed between a seriesofppressure rolls, as described in Example 2, heated at approximately 275300 F. This compacted the plies and .made them adhere to each other. The assembly was then ypassed 'through a heat zone having an air temperature of e700-800 F., at atmospheric pressure, where the impregnant was heated to at least its .fusion ktemperature caf-.62.1 .F.

After the .laminated assembly was cooled, it was cut into suitable lengths having substantially yno voids in the ent-ire structure. There were nofmud cracks in the fused polymeric material, and the .individual plies Vhad been vrtnly bonded into .an integral unit which could not be separated .into its original components.

AExample f6 Two more .continuous lengths or plies were cut from the dry, un'fused, impregnated and .ca'lenderedglass fabric described yin Example `3 tand were arranged in superposed relation with the 'end of a .continuousllength of a single ply .of the dry, unfused, impregnated, and caiendered nonwoven ,asbestos ,fabric described .in Example 5 interposed therebetween. The assembly was .then passed through a series of -pressure rolls, as described in Example 2 heated at approximately 275 `30il `F. The assembly was then passed through .a heat yZone having an air temperature of 700-800`D P., where the inipregnant was heated to at least its fusion temperature of .621o F.

Example 7 A continuous length of woven asbestos fabric, containing -'85% asbestos (ASTM specification D299- 49T) and yhaving 'the followingcharacteristics:

Thickness (inches) .025 Ounces per square yard 8.0 rlhread count per inch:

Warp 22 yFiller l5 was given three dip 'coats of a 50% polytetra'lluoroethylene aqueous suspensoid which thoroughlyimpregnated the fabric. The saturated fabric was then dried at 250 F. -The weight ofthe dry ypolymer deposited in the fabric was approximately 23 ounces vper square yard. The average thickness of the lwoven asbestos fabric after impregnating and drying was .040 inch.

The continuous length .fof -dr-y, unfused, 4impregnated asbestos fabric twas cut into two 'pieces which were then arranged 'in superposed relation, andthe whole assembly was -passe'd "through/a series :of pressure rolls, fas described in Example 2, heated at approximately 275%-3001 F.

7 This compacted the plies and made them adhere to each other. The assembly was then passed through a heat Zone having an air temperature of 700-800 F., where the impregnant was heated to at least its fusion temperature of 621 F.

After the laminated assembly was cooled, it was cut into suitable lengths having substantially no voids in the entire structure. The product was a continuous-length, firmly-bonded, semirigid, unitary structure about .045 inch thick.

All the above examples produced unitary structures which were capableV of withstanding severe abrasion, which were substantially free of voids or air spaces, and in which the fabric plies were completely encased within the mass of polymeric material. vThis latter feature increases the cut-through resistance of the polymeric material and thus prevents loss of insulation when a metal conductor is pressed thereagainst.

It will be obvious that copolymers of tetrafluoroethylene with one or more polymerizable organic compounds containing an ethylenic double bond, such as ethylene, vinyl chloride, vinylidene chloride, and alkyl esters of acrylic and methacrylic acids may be used in place of the homopolymer. When such a copolymer is used, it is preferred to use a copolymer of tetrafluoroethylene and ethylene, and particularly one which contains from 60% to 85% tetraliuorethylene and 40% to 15% of ethylene.

The minimum temperature at which fusion of poly'- tetrafluoroethylene occurs is 621 F., and that at which fusion of the copolymer occurs is dependent on the proportion and fusion point of the modifying material or materials present, and will be somewhat less than 621 F. The maximum temperature for either type of polymerio material is just below that at which undesirable decomposition occurs.

As shown in Example l, it is not essential to heat the steel pressure roll during the rolling of the superposed plies to form the compacted laminate. However, when the plies are rolled or pressed together at room temperature, thhe polymeric material does not adhere to itself as tenaciously as it does if the pressure is applied with a slightly elevated temperature, and it is therefore necessary to exercise greater care in handling the material to prevent the plies from becoming separated. For this reason, it is preferable to heat the rolls to about 325 F.

Although not essential in the practice of this invention, it is preferred that the dry, unfused, impregnated fabric be calendered before laminating, as described in Examples 2 and 3. It is also possible to use impregnated fabric in which the impregnant is fused before laminating, and then fused again after being laminated as described herein.

Various combinations are possible in carrying out this invention: all of the plies may be fused or unfused prior to passing them, in superposed relation, through the pressure rolls; or there may be alternate plies of fused and unfused; or one or more outer plies may be fused and one or more intermediate plies may be unfused, or vice versa; furthermore, it is possible to laminate an unsupported film of fused or unfused polytetrafluoroethylene with one or more plies of fused or unfused impregnated fabric.

The examples show laminations of two and three plies. It will be obvious, however, that structures of more than three plies may be produced by the process of this invention. For example, more than three plies may be passed through the rolls at one time, or additional plies may be rolled together, one at a time, before the entire assembly is subjected to the fusion temperature.

lt has been found that the dielectric strength per mil thickness of the laminates of this invention increases with the vnumber of separate plies used'.

In the preferred embodiment of this invention as shown in Figs. land 2, the fusion operation is carried out immediately -following the rolling or pressing' together of the separate plies. However, it-is possible to roll .0r pressv the separate plies together and then transport them to another location for the subsequent fusion operation, there being no limit on the amount of time that may elapse between the rolling or pressing together of the separate plies and the fusion operation. However, care must be exercised in handling the coherent compacted plies before the fusing operation since they are not firmly bonded at this stage, and any separation of the plies before the fusing operation which would result in air pockets or voids in the laminate and loss of dielectric strength should be avoided.

The method of this invention is useful for laminating polymeric material impregnated substrates other than those fabrics described in the examples, such as nonwoven glass fabric, woven asbestos fabric, and metal (woven wire) fabric, the only limitation being that the substrate must be able to withstand the high temperature required for the final fusing of the polymeric material. Siliceous fabrics, and, particularly, glass fabrics, are preferred because of their physical and electrical properties.

Flexible laminates made by this invention may be readily formed into slot liner insulation. Such slot liners have the much desired snap-back of sh paper slot liners; they do not lose their good dielectric strength at the crease; they are slippery, which makes for easy loading and more complete filling of the slot cell space; cold-flow and cut-through troubles are minimized; they have excellent mechanical strength and abrasion resistance; and they have a heat resistance of Class H (American Institute of Electrical Engineers scale for heat resistance of electrical insulation).

It is apparent that many widely different embodiments of this invention may be made without departing from the spirit and scope thereof; and, therefore, it is not intended to be limited except as indicated in the appended claims.

l claim:

l. A continuous process of producing a unitary structure which comprises arranging in superposed relation a plurality of continuous plies of woven fabric which have previously been impregnated with a polymeric material comprising tetrauoroethylene units in preponderant amount and dried, lightly pressing successive portions of the assembled plies to form a coherent structure, and subsequently heating successive portions of the coherent structure at a temperature above the fusion point of the polymeric material in the absence of any applied pressure to weld all of the polymeric material together into a continuous length of laminated material, the fabric being one which will not decompose under the conditions recited. v

2. The process of claim l in which the impregnated fabric plies are calendered before being arranged in superposed relation.

3. The process of claim l in which thc fabric is glass.

4. The process of claim l in which the fabric is asbestos.

5. The process of claim l in which the fabric is wire.

6. The process of claim l in which the pressing is carried out at a temperature below the fusion temperature of the impregnant.

7. The process of claim 1 in which the pressing is carried out at 325 F.

8. The process of claim l in which the impregnant is polytctrauoroethylene.

9. The process of claim 8 in which the impregnant of the fabric plies is unfused when the plies are arranged in superposed relation.

l0. The process of claim 8 in which the impregnant of the fabric plies is fused when the plies are arranged in superposed relation.

1l. The process of claim 8 in which alternate fabric plies have the impregnant fused and unfused when the plies are arranged in superposed relation.

l2. The continuous process of producing a unitary structure which comprises impregnatingl a continuous woven fabric with a colloidal suspension of a polymeric material comprising tetrauoroethylene units in preponderant amount, drying the impregnated fabric, arranging a plurality of continuous plies of the dry irnpregnated fabric in superposed relation, lightly pressing successive portions of the assembled plies to form a coherent structure, and subsequently heating successive portions of the coherent structure at a temperature above the fusion point of the polymeric material in the absence of any applied pressure to weld all of the polymeric material together into a continuous length of laminated material, the fabric being one which will not decompose under the conditions recited.

13. The continuous process of producing a unitary structure which comprises impregnating a continuous woven fabric with a colloidal suspension of a polymeric material comprising tetraiiuoroethylene units in preponderant amount, drying the impregnated fabric, calendering the dry impregnated fabric on pressure rolls, arranging a plurality of continuous plies of the dry impregnated fabric in superposed relation, lightly pressing successive portions of the assembled plies to form a coherent structure, and subsequently heating successive portions of the coherent structure at a temperature above the fusion point of the polymeric material in the absence of any applied pressure to weld all of the polymeric material together into a continuous length of laminated material, the fabric being one which will not decompose under the conditions recited.

14. The continuous process of producing a unitary structure which comprises impregnating a continuous woven fabric with a colloidal suspension of a polymeric material comprising tetratluoroethylene units in preponderant amount, drying the impregnated fabric, calendering the dry impregnated fabric on pressure rolls, heating the dry impregnated fabric to the fusion temperature of' References Cited in the lle of this patent UNITED STATES PATENTS 2,396,629 Alfthan et a1. Mar. 19, 1946 2,412,960 Berry Dec. 24, 1946 2,415,028 Bosomworth et al. Ian. 28, 1947 2,427,183 Berry Sept. 9, 1947 2,478,229 Berry Aug. 9, 1949 2,484,483 Berry Oct. l1, 1949 2,484,484 Berry Oct. 11, 1949 2,488,446 Swiss Nov. 15, 1949 2,497,712 Auchter Feb. 14, 1950 2,539,329 Sanders Jan. 23, 1951 

1. A CONTINUOUS PROCESS OF PRODUCING A UNITARY STRUCTURE WHICH COMPRISES ARRANGING IN SUPERPOSED RELATION A PLURALITY OF CONTINUOUS PILES OF WOVEN FABRIC WHICH HAVE PREVIOUSLY BEEN IMPREGNATED WITH A POLYMERIC MATERIAL COMPRISING TETRAFLUOROETHYLENE UNITS IN PREPONDERANT AMOUNT AND DRIED, LIGHTLY PRESSING SUCCESSIVE PORTIONS OF THE ASSEMBLED PILES TO FORM A COHERENT STRUCTURE, AND SUBSEQUENTLY HEATING SUCCESSIVE PORTIONS OF THE COHERENT STRUCTURE AT A TEMPERATURE ABOVE THE FUSION POINT OF THE POLYMERIC MATERIAL IN THE ABSENCE OF ANY APPLIED PRESSURE TO WELD ALL OF THE POLYMERIC MATERIAL TOGETHER INTO A CONTINUOUS LENGTH OF LAMINATED MATERIAL, THE FABRIC BEING ONE WHICH WILL NOT DECOMPOSE UNDER THE CONDITIONS RECITED. 